This invention relates in general to closing containers having noncircular openings, and more particularly to the installation of inductively heated foil membranes over the openings of such containers and to a foil membrane suitable for that purpose.
One of the most economical packages for food products consists of nothing more than a paperboard container having an initially open end that is subsequently closed by a foil membrane. Normally a snap type lid fits over the membrane and interlocks with the container rim, thereby protecting the membrane. To render the paperboard impermeable to moisture, it is coated or impregnated with a resin. The foil of the membrane is likewise coated with a resin, and these resin coatings provide an ideal vehicle for securing the membrane, for when heated, the resins will become plastic and adhere the membrane to the container rim.
One method for heating the membrane involves nothing more than bringing a heated platen down against it and conducting heat from the platen into the foil of the membrane. This is a relatively slow procedure and involves handling the membrane separately from the lid that eventually fits over it. A far superior method is induction heating. To heat the membrane inductively, the membrane while on the container is subjected to a rapidly oscillating magnetic field which produces eddy currents within the foil of the membrane. These eddy currents in turn heat the foil sufficiently to melt the resin or at least render it plastic. Depending on the strength and frequency of the field, and the thickness of the foil, the temperature rise of the membrane occurs quite rapidly, and indeed almost instantaneously. Moreover, the lid, as long as it is formed from a dielectric substance, does not affect the field, and accordingly may be in place over the container as the foil membrane is heated. In other words, the membrane may first be installed in the lid, and the lid then applied to the container, in which case the lid serves as a convenient carrier and backing for the thin and highly flexible membrane.
A wide variety of erected paperboard containers is currently available to food processors, and many are capable of accommodating heat sealed foil membranes. However, not all membranes are suitable for inductive heating, at least in accordance with present technology. It appears that inductive heating lends itself best to circular membranes applied to circular container rims of corresponding diameter. In this case, the membrane heats evenly along its periphery, thereby providing a uniform seal with the circular rim of the container.
Rectangular containers, which pack more efficiently, are not so easily sealed, at least by inductively heating foil membranes. When membranes of a generally rectangular configuration are heated inductively, they experience a substantial rise in temperature along their sides and ends, but the rise is significantly less at their corners. It appears that the eddy currents induced by the rapidly fluctuating field concentrate along the straight margins and for some reason do not extend into or are cancelled out at the corners. Any heating which does occur at the corners seems to result by way of conduction from the areas adjacent to the straight margins. Consequently magnetic fields that are adequate for securing the side and end margins, do not secure corners. Elevating the intensity of the field to the extent the corners heat sufficiently to effect a seal, scorches the membranes along their side margins, and this is unacceptable.
Machines currently exist which enable food processors to erect containers at their food processing facilities from blanks supplied by container manufacturers. This affords significant economies since it eliminates the need for shipping erected containers, which occupy considerable space, yet weigh very little. Many of these machines erect containers of rectangular cross-sectional configuration, and to complement the packing and stacking efficiences afforded by these containers, a need exists for inductively sealing foil membranes to such containers.